A solar cell or photovoltaic (PV) cell is an electrical device that converts the energy of light directly into electricity by photovoltaic effect. Energy generated from solar cellsoffers renewable, environmentally friendly and readily available alternatives to fossil fuels. Typically a solar cell utilizes semiconductor materials in the form of a p-n junction for energy conversion. Metal layers are laid on the surface of the semiconductor materials to conduct the produced voltage and current to external circuitry for power storage or transportation, e.g. through contact with a metal wire (or referred to as “front metal contact” herein) laid on top of the extraction electrode. In a typical configuration, the front metal layer of a PV cell includes a number of discrete electrodes interspersed on the front surface of the PV cell and an extraction electrode connected to all the discrete electrodes for collecting the current therefrom. An array of solar cells can be interconnected and assembled into a solar module or a solar panel to aggregate the current generated by the individual solar cells.
With the ever increasing popularity of portable electronic devices, demands for flexible solar modules have dramatically increased as they can be easily integrated into the electronic devices. Due to size and weight restriction imposed by these electronic devices, solar modules of small volumes offering high efficiency and high flexibility are preferred. It has been established that single crystalline solar cells have outstanding conversion efficiency among the large variety of PV materials that have been developed.
Unfortunately, single crystalline materials are brittle by nature and tend to break or otherwise generate defects under stress. Traditionally, a front metal contact disposed on top of a PV cell is commonly made of a metal strip (or metal wire or ribbon) which is far more rigid than a crystalline PV layer. Even very low stress can cause micro-cracks and other defects on the crystalline layer. The undesired stress may stem from lamination damage, material mismatch (e.g., coefficient of thermal expansion (CTE) mismatch), external forces, bending, and etc. Defects in a PV layer may lead to hot spots and eventually PV cell efficiency degradation. For example, a solar cell with a flexible substrate can be easily bent or otherwise deformed when subject to an external pressure, e.g., during the processes of manufacturing, assembling or installation of solar modules. When being deformed, the crystalline PV layer is subject to stress impact from the metal strip and is prone to physical damage.
FIG. 1 illustrates the crosssection of a PV device 100 along an extraction electrode of the cell in accordance with the prior art. The PV device 100 includes an encapsulation layer 110, a front metal contact 120, a front metal layer 125, a PV layer 130, a back metal layer 140 and a substrate (or carrier layer) 150. The encapsulation layer 110 is the top surface of the PV device 100 and intended to receive light beams when the PV cell is in use. The front metal layer 125, the PV layer, the back metal layer 140 and the substrate 150 are collectively referred to as the PV cell. The front metal layer 125 corresponds to an extraction electrode (or known as a “bus bar”) of the PV cell. For example, the front metal layer 125 is formed by electroplating. The front metal contact is disposed on top of the front metal layer 125 and made of a metal strip composed primarily of Cu.
According to the conventional front contact metallization approach, the front metal contact is in direct contact with the PV cell. The PV layer is fabricated as a crystalline thin film structure and is less rigid than the metal contact. The mismatch in material rigidity between the PV cell and the front metal contact tends to cause defects or other damages in the PV layer 130, especially the area under the extraction electrode due to its greater size than the discrete electrodes.